油井日产油量与油井采油压差之比，称为（）。
A.采油强度
B.采油指数
C.采油速度
D.采出程度
参考答案：B
The coal miners in the coal mine for 10 hours before we came to their rescue.
A．had trapped
B．had been trapping
C．had been trapped
D．would have been trapped
摩擦阻力的大小主要取决于接头螺栓（）和钢轨与夹板接触面之间的摩擦系数。
